High-speed and high-capacity transmission is required at a distance of several meters to several tens of meters, for example, between servers in a data center, and between devices of a large scale computer (supercomputer) network. In optical communication, a transmission rate of 25 Gbps (gigabit per second) and a transmission capacity of 100 Gbps (25 Gbps×4) are realized, and data transmission between devices is transitioning from an electrical communication to an optical communication.
For the optical transmission between devices or between processors, active optical cables are used instead of electrical cables. An active optical cable is an optical transceiver in which an E/O (Electrical-to-Optical) conversion module for converting an electrical signal into an optical signal, an optical fiber for transmitting an optical signal and an O/E (Optical-to-Electrical) conversion module for returning the optical signal received from the optical fiber to an electrical signal are integrated. A connector part of the active optical cable connected to a computer is compatible with a connector of the conventional electric cable. High-speed and large-capacity transmission by optical communication becomes possible merely by connecting the active optical cable instead of the electric cable to the computer.
A VCSEL (Vertical Cavity Surface Emitting Laser), which is low in cost and power consumption, is being widely used as a light emitting element in an E/O module. A CML (Current Mode Logic) driver is used to drive a light emitting element such as a VCSEL at a high speed in optical transmission of a relatively short distance. A CML is used in a differential high-speed interface because its configuration is simple and its rise/fall speed (edge rate) is high.
A technique has been known that the output of a differential amplifier is returned by a current mirror circuit so that the final stage transistor is operated at a high speed, the power supply voltage is lowered without deteriorating an optical output waveform, and the power consumption is reduced.
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2000-124862.